The present invention relates to digital moire interferometry, and particularly to a system and method using a calibrated volume to provide absolute measurements of distance from digital moire phase information obtained from an object under test, regardless of phase ambiguities in such information.
The invention is especially suitable for use in machine vision systems for measuring the contour of three dimension objects through selected cross-sections thereof. Such measurements may be used for inspection of objects as well as for defining their location in space.
Digital moire techniques are known where a line grating is projected on an object under test. Digitized video information is obtained with a camera viewing the object under test at an angle with respect to the axis of the optics which project the grating. Phase information is derived from the digitized video using a spatial synchronous algorithm (by convolving the digitized video information from the camera with orthogonal reference sine and cosine functions). See K. H. Womack, "Interferometric Phase Measurement Using Spatial Synchronous Detection", Optical Engineering, Vol 23 No. 4, Page 391, July/August 1984. Phase information obtained as a result of such measurements is only relevant with respect to a reference plane and becomes ambiguous at distances greater than a half period of the projected grating as it is viewed at the camera. Techniques for removing the ambiguity have involved taking into account the geometric relationship of the projector and the camera (triangulation) to obtain a reference plane and stepping of the grating to different positions along the path of the light projected on the object in order to obtain phase information representing the profile of objects which are large with respect to the grating. See U.S. Pat. No. 4,641,972 issued Feb. 10, 1987.
Other schemes for resolving the ambiguity in making contour measurements have involved use of light of different wavelengths, synchronously moving the object under test and the projector and/or camera optics or providing other special geometric orientation of the optics. Various examples of such schemes are described in the following patents, which deal with contour measurements using interferometry techniques, and which also describe complex computational processes used in deriving the phase information from the video signals: U.S. Pat. Nos. 4,456,339 issued June 26, 1984; 4,498,770 issued Feb. 12, 1985; 4,641,971 issued Feb. 10, 1987 (the last three patents involve multiple frequency systems); 4,131,365 issued Dec. 26, 1978; 4,391,526 issued July 5, 1983; 4,457,625 issued July 3, 1984; and 4,627,734 issued Dec. 9, 1986 (the last four patents involve moving optics/object systems); 4,009,965 issued Mar. 1, 1977; 4,139,304 issued Feb. 13, 1979; 4,340,306 issued July 20, 1982; and 4,387,994 issued June 14, 1983 (the last four patents involve contour measurements using spatial orientations of the projection and detection optics).
In the various systems and techniques which have heretofore been proposed, measurement accuracy is limited by the alignment of the various optical elements (the projectors and the cameras or other photoelectric detectors), aberrations in the lenses and the proximity of the reference plane to the surface of the object under test from which the projected light is reflected. In addition, the measurements provided by such systems are only the relative phase with respect to the reference plane.